Two-mica and tourmaline leucogranites from the Everest-Makalu region (Nepal-Tibet).: Himalayan leucogranite genesis by isobaric heating?

In the Higher Himalaya of the region from Cho Oyu to the Arun valley northeast of Makalu, the Miocene leucogranites are not hosted only in the upper High Himalayan Crystallines (HHC) a network of dykes also cuts the lower HHC and the Lesser Himalayan Crystallines (LHC). The plutons and dykes are mainly composed of two-mica (muscovite+/-biotite +/-tourmaline+/-cordierite+/-andalusite+/-sillimanite) leucogranite, with tourmalineless than or equal to52.6% and biotite>1.5% modal, and tourmaline (muscovite+/-tourmaline+/-biotite+/-sillimanite+/-garnet+/-kyanite+/-andalusite+/-spinel+/-corundum) leucogranite, with tourtnaline>2.2% and biotite<1.5% modal. Both leucogranite types were produced by partial melting in the andalusite-sillimanite facies series, under LP/HT conditions constrained by the occurrence of peritectic andalusite and cordierite. The geochemical features of the leucogranites suggest that tourmaline leucogranite was produced by muscovite dehydration melting in muscovite-rich metapelites at Psimilar to350 MPa and Tgreater than or equal to640degreesC, whereas two-mica leucogranite was produced by biotite dehydration melting in biotite-rich metapelites at Psimilar to300 MPa and Tgreater than or equal to660-710degreesC. Melting in fertile muscovite-rich metapelites of the top of both the HHC and LHC produced magmas which were emplaced at the same structural level in which they had been generated. Melting in the biotite-rich gneiss of both the HHC and LHC produced hotter magmas which were transported upwards by dyking and eventually coalesced in the plutons of the upper HHC. A similar process also produced a network of two-mica granite at the top of the LHC in the Ama Drime-Nyonno Ri Range northeast of Makalu. The prograde character of leucogranite melt-producing reactions in the Everest-Makalu area suggests that, here, the generation of Miocene leucogranites took place in a regime of nearly isobaric heating following nearly adiabatic decompression. (C) 2002 Elsevier Science B.V. All rights reserved.